A reconfigurable optical add/drop multiplexer (ROADM) used in a wavelength division multiplex (WDM) system is an optical relay apparatus that has optical switches to drop and add optical signals with different wavelengths. To increase the flexibility of an optical network, implementation of Colorless, Directionless, and Contentionless (CDC) functions with which wavelengths and paths are freely set in a ROADM and wavelength contention is avoided is being studied. A ROADM having CDC functions is called a CDC-ROADM.
FIG. 1 illustrates the structure of a multicast switch (MCS) module 100 as an example of an optical switching structure that implements CDC functions. At an MCS 110-2 on the drop side, which receives WDM signals and drops optical signals, WDM signals received from M paths (deg 1 to deg M) are input to M 1×N optical couplers (represented as SPLs in FIG. 1) 131 to 13m (collectively referred to below as the optical couplers 13 at appropriate points) and are then dropped in N directions. The dropped WDM signals are connected to N M×1 optical switches 111 to 11n (collectively referred to below as the optical switches 11 at appropriate points) and are then output from M×1 optical switches 111 to 11n to N drop ports.
An MCS 110-1 on the add side, which adds, to WDM signals, optical signals to be transmitted, has the same structure as the MCS 110-2; at the MCS 110-1, optical signals are input from N add parts into N 1×M optical switches 111 to 11n. Outputs from each 1×M optical switch 11 are output to M N×1 optical couplers 131 to 13m and are then output to M paths. The 1×M optical switches 11 on the add side and the M×1 optical switches 11 on the drop side have the same switch structure; they differ only in that the number of inputs and the number of outputs are reversed depending on the signal transmission direction. In FIG. 1, therefore, each of these optical switches is indicated as M×1 SW on both the add side and the drop side. Similarly, the N×1 optical couplers 13 on the add side and the 1×N optical couplers 13 on the drop side have the same optical coupler structure; they differ only in that the number of inputs and the number of outputs are reversed depending on the signal transmission direction. In FIG. 1, therefore, each of these optical couplers is indicated as 1×N SPL on both the add side and the drop side. In this description, an optical switching structure having n output ports or input ports for m input ports or output ports will be referred to as M×N optical switch (including optical coupler, optical selector, optical splitter, and the like), regardless of the input and output directions.
In general, the MCS module 100 is used in such a way that the add side and drop side are paired as illustrated in FIG. 1. An MCS having N add ports or drop ports for M paths will be referred to as an M×N MCS.
FIG. 2 illustrates the node structure of a two-path ROADM 1001 in which wavelength selective switches (WSSs) 105a and 105b and 2×2 MCSs 110-1 and 110-2 are combined together. In a previous ROADM, arrayed waveguide gratings (AWGs) have been used at portions equivalent to the MCSs 110-1 and 110-2, so it has been possible to input only optical signal with a predetermined wavelength from each add port. However, the use of the MCSs 110-1 and 110-2 enables an optical signal with a desired wavelength to be input from each add port, so a Colorless function is achieved. This is also true on the drop side.
Optical output signals from two transponders 102 are input to signal input ports. Paths for these optical output signals are selected by a 2×2 MCS 110-1. Optical input signals from two paths are dropped to two transponders 102 by a 2×2 MCS 110-2. Since a path can be selected independently for each signal, a Directionless function is achieved. With a wavelength assigned to an input port, it is also possible to input a signal with the same wavelength from another input port (however, the same path is unable to be selected). That is, a Contentionless function is achieved.
For a CDC-ROADM node, there is a demand to increase the number (M) of selectable paths after an operation has been started. To meet this demand, at an MCS 210-2 on the drop side and an MCS 210-1 on the add side, each of 2×1 optical switches 121 to 12n (collectively referred to below as the optical switches 12 at appropriate points) are connected to one of the M×1 optical switches 111 to 11n, as in an MCS module 200 illustrated in FIG. 3. Of the 2×1 optical ports 12, N ports not connected to the M×1 optical switches 11 are collectively connected to an upgrade port 215 to increase the number of paths (see U.S. Patent Application Publication No. 2013/0108215, for example).